In U.S. Pat. No. 4,898,471, Stonestrom et al. describe a method of particle detection on a periodic patterned surface. A light collection system for collecting light scattered from the surface is arranged to maximize the particle signal compared to the pattern signal. A detector produces an electrical signal corresponding to the intensity of scattered light that is collected. A processor constructs wave form templates from the electrical signal corresponding to individual die on the surface and compares the templates to identify particles. Differences between the templates represent particles on the surface in the scan path. The reference template may be made up of registered positions of features where the detected scattering signal exceeds a preselected threshold value, so that the comparison is between corresponding positions to eliminate periodic pattern features, leaving only positions representing particles. The reference template can be constantly updated to the just compared template after each comparison, provided no particle is found, so that comparisons are mainly between adjacent die.
Such a method has been found to be useful for inspecting patterned surfaces where a relatively low particle or defect density is expected, provided the pattern on the surface generally has a single main background scattering intensity. Areas of the wafer surface where the background scattering intensity is high cannot be inspected in the same scan as the other areas and in many cases cannot be inspected at all because of saturation of the detector and because a suitable threshold cannot be found to allow inspection of the high scattering areas at the same time that inspection for very small particles and defects in the principal areas is carried out. Further difficulties occur when the particle and defect density becomes relatively large.
An object of the present invention is to provide a data processing method for eliminating the pattern features from the scattering data obtained from inspecting a patterned surface for the entire surface without also eliminating a significant number of particle and defect features from the data and while permitting inspection of high scatter regions of the surface at the same time as low scatter regions.